A superabsorbent material in general refers to a water-swellable, water-insoluble material capable of absorbing at least about 10 times its weight, and up to about 30 times or more its weight in an aqueous solution containing 0.9 weight percent sodium chloride solution in water. The present invention relates to polysaccharide superabsorbent particulate, which absorbs water, aqueous liquids, and blood, and a method to make the superabsorbent polymer and particles. The acronym SAP may be used herein, and is generally used in the industry, in place of superabsorbent polymer, superabsorbent polymer composition, superabsorbent polymer particles, or variations thereof.
A superabsorbent polymer generally is a cross linked, partially neutralized polymer that is capable of absorbing large amounts of aqueous liquids and body fluids, such as urine or blood, with swelling and the formation of hydrogels, and of retaining them under a certain pressure in accordance with the general definition of superabsorbent material. A superabsorbent polymer composition is a superabsorbent polymer that has been surface treated that may include surface cross linking, surface treatment, and other treatment. Commercially available superabsorbent polymer compositions generally include cross linked polyacrylic acids or cross linked starch-acrylic acid graft polymers wherein some of the carboxyl groups are neutralized with sodium hydroxide solution or potassium hydroxide solution. Superabsorbent polymer composition particles are particles of superabsorbent polymers or superabsorbent polymer compositions, and generally have a particle size of from about 150 microns to about 850 mircrons. A comprehensive survey of superabsorbent polymers, and their use and manufacture, is given in F. L. Buchholz and A. T. Graham (editors) in “Modern Superabsorbent Polymer Technology,” Wiley-VCH, New York, 1998.
A primary use of SAP and SAP particles is in sanitary articles, such as baby's diapers, incontinence products, or sanitary towels. For fit, comfort, and aesthetic reasons, and from environmental aspects, there is an increasing trend to make sanitary articles smaller and thinner. This is being accomplished by reducing the content of the high volume fluff fiber in these articles. To ensure a constant total retention capacity of body fluids in the sanitary articles, more SAP content is being used in these sanitary articles.
Most of the superabsorbent materials used today are primarily based upon slightly crosslinked synthetic polymers. These include, for example, polymers, and co-polymers based upon acrylic acid or acrylamide, which are not based upon renewable materials and are insufficiently or not at all biologically degradable.
The present invention generally relates to a process for producing a polysaccharide superabsorbent polymer particulate, a polysaccharide superabsorbent polymer particulate obtainable by this process, or the use of the polysaccharide superabsorbent polymer particulate. Numerous water-absorbing polymers are described in the literature that are based upon polysaccharides, and that are at least partially biodegradable. The raw materials for the production of superabsorbent polymers based upon polysaccharides are, however, frequently water-soluble and must be converted into the water-insoluble form, in order to be able to use them as superabsorbers for hygiene applications.
EP 0 538 904 A1 and U.S. Pat. No. 5,247,072 describe superabsorbers based upon carboxyalkylpolysaccharides. In the process, the carboxyalkylpolysaccharide is dissolved in water and isolated by drying or precipitation and then thermally crosslinked via internal ester bridges by the reaction of the hydroxyl groups of the polysaccharide skeleton with the acidic carboxyl groups. Since this crosslinking reaction is very sensitive to small changes of the pH value, the temperature or the reaction duration, absorbers with widely varying absorption properties are obtained. The materials are characterized by a high absorption capacity under pressure, which, however, falls to a fraction of the original absorption properties within a few weeks, upon storage of the absorber.
U.S. Pat. No. 5,550,189 describes superabsorbent polymers based upon carboxyalkylpolysaccharides, in which the aging stability is improved by addition of multifunctional crosslinkers, such as, e.g. aluminum salts or citric acid. The production of the absorbers occurs from a common, homogeneous aqueous solution of carboxyalkylpolysaccharide and crosslinker, in which the components are present in low concentration, isolated together, and then thermally crosslinked. The synthesis of these absorbers requires a high energy and time consumption, since the aqueous solutions are only of very low concentration. The improvement of the aging stability in the many exemplary embodiments does not correspond to the demands relevant in practice.
One aspect of the present invention is to overcome the disadvantages arising from the state of the art. It is a goal of the present invention to provide biodegradable superabsorbent polymers based upon renewable raw materials, which do not have the above described deficiencies. Such superabsorbent polymers should have a high long-term storage stability, in which the absorption properties remain as far as possible. At the same time it is intended that the absorber particles have a high mechanical stability, in order to avoid the formation of fine dust parts during processing processes such as, for example, sieving or conveying.
A further aspect of the invention finds a production process for such superabsorbent polymer, which is simple, economical and may be reliably carried out, delivers a uniform product quality, and in which small quantities of solvents are used and organic solvents are avoided if possible. Furthermore, it should be possible to carry out the process without the use of toxicologically questionable substances.